1. Field of the Invention
The present invention relates in general to multiple antenna impedance optimization. In particular, the present invention relates to a method and apparatus for impedance transformation between two antennas in close proximity to each other.
2. Background
Cellular radiotelephones, combined cellular and satellite radiotelephones, and other wireless communications devices often employ two or more antennas, each of which are connected with a separate radio. Due to the limited space on most wireless devices, it is highly desirable to locate these antennas close together. However, without isolating the electromagnetic coupling between the antennas, there is a limitation on how closely the antennas can be spaced from each other. Coupling between the antennas creates several problems, including: reducing the gain of each antenna because some of the radiated power from each antenna is absorbed by the other antenna; creating tuning and impedance mismatches in each antenna, causing mismatch loss and/or lower impedance bandwidth; mixing of signals which can result in spurious emissions; and damaging of a receiver of one radio by a strong signal transmitted from the other radio.
Multiple antenna isolation can be achieved by placing a circuit in series between the radio transmitter and its antenna. Examples of series circuits are filters, switches, and directional attenuators. A series filter circuit presents a lower insertion loss across the frequency band of the first antenna and a higher insertion loss across the frequency band of the second antenna. A switch is closed when its antenna is in use and open when the second antenna is in use. The switch should be located near the base of the antenna to ensure that the length of transmission line between the switch and the antenna base does not transform the open circuit impedance at the switch to some other impedance as described in U.S. Pat. No. 5,060,293. A filter in combination with a directional attenuator provides antenna isolation as described in U.S. Pat. No. 5,815,805. A shortcoming of filters is the insertion loss, which can be significant. A shortcoming of using a switch is that the switch must be located very close to the base of the antenna.
Multiple antenna isolation can be achieved by creating a canceling signal (interference signal) in a third antenna that cancels the signal from the second antenna, as described in U.S. Pat. No. 4,233,607. This method requires additional hardware including an antenna and a signal generator signal to generate the canceling signal. Multiple antenna isolation can also be achieved by anti-phase combination of signals as described in U.S. Pat. No. 5,264,862. Multiple antenna isolation can also be achieved by using uncorrelated radiating modes as described in Canadian patent 2,095,304. Using uncorrelated radiating requires the two antennas to be oriented in one of a limited number of possible orientations to create orthogonal polarization and radiation patterns. Such limited orientations prohibit using this method in many applications with physical space constraints. Further, this method can be applied to at most three antennas. Multiple antenna isolation can also be achieved by arranging narrow beamwidth antennas sectorally such that their radiation patterns do not overlap as described in U.S. Pat. No. 5,771,449. However, sectoral arrangement is impractical in most applications with size constraints, such as cellular telephones.
A wide band antenna can be used with a frequency diplexing circuit to separate the communication signals into the appropriate frequency bands. For example, a single antenna in a cellular telephone can be used to simultaneously transmit and receive cellular telephone calls. These designs have several disadvantages. First, a single feed point wide band antenna with multiple radios attached is difficult to design. Second, the frequency diplexing circuit exhibits high insertion loss. Higher insertion loss causes lower communication quality and higher battery current consumption rates, which decreases the operational time in battery operated devices.
Alternatively, a multiple pole switching circuit can separate transmit and receive frequency ranges on a wide band antenna. The multiple pole switching circuit has three primary disadvantages: high insertion loss, increased current consumption, and lower linearity. Lower linearity is a result of an increase in spurious emissions during transmitting and an increase in spurious input signals during receiving.
A dual-mode phone operates on two modes, usually digital and analog. For example, a dual-band phone operates on the cellular band (800 MHz) and the PCS band (1900 MHz).
A brief summary of the mobile standards commonly used includes:
Multiple access techniques: FDMA allows multiple stations to use different frequencies within an operating frequency channel. Time Division Multiple Access (TDMA) allows mobile stations to use the same frequency, but signals are separated by time slots. Code Division Multiple Access (CDMA) allows multiple mobile stations to use the same frequency, but signals are separated by unique digital codes. CDMA uses spread spectrum techniques. Personal Communication Services (PCS) is a digital communication standard that is commonly referred to as the 1900 MHz (1.9 GHz) band. However, the band is actually from 1850 MHz to 1990 MHz.
Operating modes that use one or more multiple access techniques: Advanced Mobile Phone System (AMPS) is an analog system used in the United States for cellular telephones. AMPS uses Frequency Modulation (FM) and the FDMA air interface. The frequency band for AMPS is 824 MHz to 849 MHz and 869 MHz to 894 MHz. Each channel is 30 KHz wide. Narrow-band Advanced Mobile Phone Service (NAMPS) operates with the 30 KHz channels used in AMPS divided into three 10 KHz channels. Global System for Mobile Communications (GSM) is a European standard for digital wireless communications. GSM uses a combination of FDMA and TDMA. GSM divides the 25 MHz band into 124 frequencies of 200 KHz each. GSM uses 8 time slots rotated at 214 times per second. GSM in the United States uses the PCS band (1900 MHz). Digital Advanced Mobile Phone System (DAMPS), like GSM, uses TDMA and FDMA. However, DAMPS uses 3 time slots rotated at 50 times per second. Bluetooth is a specification for short range radio links between mobile PCs, mobile phones and other portable devices. Bluetooth radios operate in the unlicensed ISM band at 2.4 GHz and use a time-division duplex scheme for full-duplex transmission. The range of Bluetooth is only from 10 cm to 10 m, but can be extended to 100 m. Thus, Bluetooth is useful as a data link between a cellular telephone and a near by computer. Mobile satellite telephones, communicate via satellites instead of cellular base stations. Such phones are available from IRIDIUM and GlobalStar.
FIG. 1 shows a typical prior art multiple antenna system 100 with two radio antenna systems 102, 104 that uses series circuits. The radio antenna system 102 includes a radio 110, an antenna 114, and a series circuit 112, in series between the radio 110 and antenna 114. The radio antenna system 104 includes a radio 120, an antenna 124, and a series circuit 122 in series between the radio 120 and antenna 124.